Precessional cyclicity of seawater Pb isotopes in the late Miocene Mediterranean

Citation

Abstract

Astronomical tuning in the Mediterranean region is primarily based on organically-mediated proxies, such as cyclicity of organic rich layers or changes in foraminiferal assemblages. Both during and post deposition, organic proxies can be affected by complex processes not immediately related to the changes in precession (insolation) they are assumed to reflect. Here we present an isotopic proxy which exhibits precessional cyclicity yet is inorganic. Seawater Pb isotope records over four precessional cycles between 6.6 and 6.5 Ma, from bulk sediment leachates of three Messinian, circum-Mediterranean marginal locations, show variations consistent with precessional cyclicity. During insolation minima, the Pb isotope signatures from all three sites converge to similar values, suggesting a regional process is affecting all three locations at that time. Data from the marginal sites are compared with new data from ODP Site 978 and published data from a variety of geological archives from the Mediterranean region to determine the mechanism(s) causing the observed variability. While the comparisons are not fully conclusive, the timing of events suggest that increased dust production from North Africa during insolation minima is the most likely control. This hypothesis implies that authigenic marine Pb isotope records have the potential to provide a reliable inorganic tie point for Mediterranean cyclostratigraphy where sub-precessional resolution is required. An inorganic tie point could also provide the means to resolve long standing problems in Mediterranean stratigraphy on precessional and subprecessional timescales which have been obscured due to post-depositional changes (e.g., sapropel burn-down) or suboptimal ecological conditions (e.g., the Messinian Salinity Crisis).